CHU, the Canadian shortwave time station near Ottawa, is similar
to WWV in the US but with one important difference: in addition
to announcing the time in both French and English, it also
broadcasts the current time once per minute using the old "Bell
103" (300 baud) modem tones. These tones are very easy to
decode, and Bill Rossi realised that you don't even need a
modem-- just a shortwave radio and a sound card. If you're able
to receive the signal from CHU, this may be the cheapest radio
clock available. Shortwave reception varies throughout the day,
but Bill claims that by changing frequencies twice a day (morning
and evening) he gets almost 24-hour coverage. CHU broadcasts on
3.33, 7.335, and 14.670 MHz.

The NTP website has plans for a "gadget box" that
decodes the CHU time broadcast using an inexpensive 300 baud
modem chip and any shortwave radio, at
http://www.eecis.udel.edu/~ntp/ntp_spool/html/gadget.htm.
The plans include a Postscript image of a
2-sided custom printed circuit board, but you have to make the
board yourself (or find someone who can make it for you).

Ntpd includes a driver (type 7) for CHU receivers, which
works either with modems like the "gadget box",
or by feeding the audio directly into the mic input of a Sun SPARCstation
(or any other machine with "compatible audio drivers").

You may have heard about Heathkit's "Most Accurate Clock",
which received and decoded the time signal from WWV and had an
optional serial port for connecting to a computer. Heathkit
stopped selling kits a long time ago, but they continued to sell
the factory-built version of the clock until 1995, when it was
also discontinued. For Heathkit nostalgia (not including the
clock) see
http://www.heathkit-museum.com. The
Heathkit company still exists, selling educational materials. See
http://www.heathkit.com.

According to Dave Mills, Heathkit's patent on the "Most Accurate
Clock" is due to expire soon, so maybe someone out there would
like to clone it as a single-chip IC.

The NTP website has a DSP program (and a PDF file describing it) at
http://www.eecis.udel.edu/~mills/resource.htm
that decodes the WWV time signal using a shortwave radio and the
TAPR/AMSAT DSP-93, a DSP kit which is no longer available. It was
based on the Texas Instruments TMS320C25 DSP chip. The TAPR
website at
http://www.tapr.org includes a lot of
information on homebrew DSP programming.

Ntpd includes a driver (type 6) for the IRIG-B and
IRIG-E time codes, using /dev/audio on a
Sun SPARCstation, with a note that it is "likely portable
to other systems". WWV uses the IRIG-H time code.

GPS signals include the correct time, and some GPS receivers have
serial ports. Ntpd includes drivers for several GPS
receivers. The 1PPS feature ("One Pulse Per Second",
required for high accuracy) usually requires a separate interface
to connect it to the computer.

TAPR (Tuscon Amateur Packet Radio) makes a kit for an interface
called "TAC-2" (for "Totally Accurate
Clock") that plugs into a serial port and works with any
GPS receiver that can provide a 1PPS output-- including some
"bare board" models that can be mounted directly to
the circuit board. For more information see their website at
http://www.tapr.org. The price (as of June 1999) is around
$140, not including the GPS receiver. The kit does not
include any enclosure or mounting hardware.

These low-frequency stations broadcast a time code by simply
switching the carrier on and off. Each station uses its own
coding scheme, and summaries are available on the NTP website at
http://www.eecis.udel.edu/~mills/ntp/index.htm
(near the bottom of the page). DCF77 in Germany broadcasts on
77.5kHz. MSF in England (also called "Rugby", which
apparently refers to its location) and WWVB in Colorado both
broadcast on 60 kHz.

Inexpensive receivers that can plug into a serial port are
reported to be available in Europe. Ntpd includes
drivers for a couple of MSF receivers.

A number of companies in the US sell relatively inexpensive
clocks that have built-in WWVB receivers (including several
analog wall clocks), but I'm only aware of two that can be
connected to a computer:

The Ultralink Model 320 sells for about $120 (as of June
1999) and has a serial interface and a straightforward ASCII
command set, so it shouldn't be too hard to program. It draws 1mA
from the serial port for power. The antenna can be up to 100 feet
away from the computer, and the unit contains its own clock to
maintain the time if it loses the signal. They also sell a
"bare board" version for about $80 that is
designed to work with the "BASIC Stamp" series of
microcontrollers. See
http://www.ulio.com/timepr.html.

Arcron Technology sells a desk clock with an optional serial
port for about $130, including software for Windows. See
http://www.arctime.com